This disclosure generally relates to systems and methods for spectral analysis and, more particularly, to systems and methods for hyperspectral imaging.
Hyperspectral images are a category of digital color images. A digital image is represented in a computer by a two-dimensional array of elements. For a monochrome image, each element is a scalar representing intensity of light. For a color image, each element is a vector of two or more scalar values representing intensity at various wavelengths of light. In the color images used in common computer systems and cameras, each element has three scalar values of intensity: one each for red, green, and blue wavelengths (see FIG. 1A) or one each for cyan, yellow, and magenta wavelengths. In a hyperspectral image, each element has more than three values—sometimes many more (see FIG. 1B). Such an image provides fine-grained spectral data that can be useful in quality control (especially of paint coatings), agriculture, oceanography, oil and mineral prospecting, pollution tracking, medical diagnostics, and military surveillance.
One reason hyperspectral images are not used more often is that these images are typically created using hyperspectral cameras. Unlike a conventional color camera which applies three filter colors to each pixel, a typical hyperspectral camera uses dozens of filter colors, or it uses a diffraction grating at each image location to scatter light of different colors onto dozens of separate elements of a charge coupled device (hereinafter “CCD image sensor”). There are many types of hyperspectral camera. In one type, a filter wheel is configured to position several narrow-band filters in front of a single camera. That filter wheel is positioned by an electromechanical actuator. Another type of hyperspectral camera includes a mechanically variable interferometer disposed in front of a single camera and an electromechanical actuator. A third type is a line-scan imaging spectrometer that uses a strip of diffraction grating (or prism) to scatter light from a slit aperture onto a two-dimensional array of sensor elements. The device must be scanned by rotation or translation to cover a spatial two-dimensional scene. It also contains an electromechanical actuator, which reduces the reliability of an aircraft mission system. Yet another type comprises multiple cameras facing the same direction, each with a different filter in front of it. The foregoing complications make hyperspectral cameras heavy, bulky, and expensive.
Without access to a hyperspectral camera, some systems rely on ordinary color cameras or even monochrome cameras. Data from these are supplemented by other systems, such as a thermal imaging camera, radar, or measurements involving physical contact. These other systems are typically heavy and expensive.
It would be advantageous if hyperspectral images could be acquired using a small, cheap camera, such as a conventional color camera, without the need for supplemental data from another system.